Obesity is defined as an excess of body fat.
That is an abstract idea, but what is its operational definition? You can
start off by asking people to define themselves as being obese or not, but this
invokes subjective cultural norms that may shift over time. In the late
twentieth century, skinny models such as Twiggy and Kate Moss are considered to
be chic in the western world. A century or two ago, the ideal west woman is much more rotund. In Chinese history, there are two famous
beauties, one was rather corpulent and the other renowned for her tiny
waist. And then an anorexic will probably consider anything to be
overweight.

It is preferable, therefore, to have an objective measure of
obesity. If being obese means being overweight, then surely weight has to
figure in this measure somewhere. But consider a person who weighs 70
kilograms. Is that overweight or not? Well, that depends on the
height of the person. For a very short person, that amount of weight
requires a wide girth to carry it. But a very tall person may look quite
gaunt at that weight. Therefore, height should also figure somewhere in
the measure (but be careful about specifying that the height should be measured
'without shoes on'!).

The most commonly accepted guideline based on weight and
height to determine underweight and overweight is the Body Mass Index, defined
to be:

After calculating the BMI, the following classifications are
often used:

BMI less than 20, which means a low amount of body
fat. This is fine for an athlete in training. For a non-athlete,
the weight is probably too low and the associated frailty lowers immunity to disease.

BMI between 20 and 25 is the normal range

BMI between 25 and 30 is considered overweight and carries
some risk for a number of illnesses

BMI over 30 is considered an unhealthy condition, with high
risk for diabetes, high blood pressure, hypertension, heart disease and so
on.

We will now look at some anthropometrical measures from the TGI
Brasil study. This is a survey of 10,624 persons between the ages of 12 to
64 years old conducted during year 2002. During the survey,
the information about height and weight is taken, from which the BMI is
calculated. According to the TGI Brasil study, the mean BMI is 24.1, which
puts the average Brazilian at the upper end of the 'normal' range.

The first question that we ask is this, "Are rich people
obese?" According to the TGI Brasil, the mean BMI among the
socio-economic classes of AB, C and DE are respectively 24.1, 24.1 and 24.2
respectively. Therefore, we see no significant differences in mean BMI by socio-economic
class. But if we look at the percentages of people who have BMI over
30, then the number is 11.0% in the total sample, and 9.8%, 11.1% and 12.2% in
the AB, C and DE classes respectively. So the answer to the original question is
"No" and in fact rich people are less likely to be obese. Today, there is no mass
starvation in Brazil, but the nutrition and diets are still far from ideal for
the poor, and the excess of starch and fat plus the lack of exercise among the
poor cause more
incidences of obesity.

The next question relates to the incidences of obesity by
age/sex groups. This is where BMI breaks down. For children who are
still growing, the bones grow faster than the body fat could keep up, so that
children have a lower BMI which picks up as they grow older. In the next
chart, we show the relationship between BMI by age/sex groups. Therefore,
given this type of systematic pattern, it is obviously important to go beyond
the single classification scheme and establish age-specific Body Mass Index
classifications.

(data source: TGI Brasil)

The moral of the less here is that the haste to find a
quick-and-easy quantitative measure should not blind people to the presence of
systematic deviations that invalidate the simple measure. Can you think of any
other exceptions? Well, how about pregnant women?